1. Field of the Invention
This invention relates to a method of producing permalloy cores.
2. Description of the Prior Art
High permeability Ni--Fe magnetic alloys are widely used to form magnetic cores for light electrical equipment applications, and as a magnetic shielding material. The permalloy cores (soft magnetic Ni--Fe alloy cores) are produced by slitting sheet of a prescribed thickness to a final width, shaping the strips in accordance with the intended usage and following this by annealing.
Cores are divided broadly, by the processing method used, into wound cores and punched cores. Wound cores are produced by winding strips slit to a final width into cores having a prescribed inside and outside diameter. To produce punched cores, core pieces of a prescribed shape are punched or stamped out of strips slit to a final width. The usual shapes are that of an "E," "I" or "U," and the punched pieces are stacked to form stacking cores. The punched pieces may also then be subjected to bending and drawing for final application as magnetic shielding material or the like.
The permalloy cores thus formed are subjected to magnetization annealing at 900.degree. to 1300.degree. C. to impart the prescribed magnetic properties. The high temperature can cause the core pieces to seize together or to be burned at points of contact with the metal vessel. To prevent this, taking wound cores as an example, after being slit to the prescribed final width the core strips are immersed in a slurry of water and alumina (Al.sub.2 O.sub.3) or magnesia (MgO), dried, wound and subjected to magnetization annealing. In the case of punched cores, after the core pieces have been punched out and the punching fluid removed, the cores are then coated with finely powdered alumina or magnesia or the like to prepare them for the annealing.
Thus, in the prior art methods the core pieces or core strips have to be subjected to complex, inefficient processes such as degreasing, annealing separator application and drying and the like. Moreover, in the case of wound cores the thickness of coatings can vary in a core and from core to core, which can readily lead to non-uniform pressure during the winding operation and degrade the magnetic properties of the end product. Also, because in the case of punched cores the size of the cores are usually small, in practice coating is a very complex procedure.
One solution to this problem has been proposed by JP-A-63-202009, in accordance with which a spray gun or supersonic wave means or the like is used to spray on a thin film of a colloid of boric acid, magnesium oxide (MgO), sodium silicate and water before the cores are formed, which, particularly in the case of punched cores, prevents the core pieces sticking together and therefore eliminates the conventional need to separate the core pieces one by one. With respect also to wound cores, there is a disclosure that should enable annealing efficiency to be increased by making it easier for hydrogen gas to enter between layers of the winding during the annealing.
While such methods are effective with respect to making it easier for punched core pieces to be separated and for enabling gas to be introduced and removed during annealing of wound cores, they still have the following basic drawbacks. Because both surfaces are coated with a thin film of a ceramic abrasive such as boric acid, MgO and sodium silicate, there tends to be considerable damage to punches and molds, which is very uneconomical with respect to the machine tools and in practice makes such methods difficult to use. With respect also to the spray method, it is difficult to apply the coating stably for an extended period which makes the securing of highly stable magnetic properties problematic.